Phase inverter for push-pull amplifiers



July 8, 1941. E w. D. CANNON v 2,248,852

PHASE INVERTER FOR PUSH-PULL AMPLIFIERS Filed April 12', 1940 PatentedJuly 8, 1941 Fries PHASE INVERTER FOR PUSH-PULL AMPLIFIERS William D.Cannon, Metuchen, N. J., assignor to The Western Union TelegraphCompany, New York, N. Y., a corporation of New York Application April12, 1940, Serial No. 329,350

4 Claims.

This invention relates to electric signaling transmission systems andmore particularly to electron discharge amplifier circuits adapted tojoin an unbalanced type of amplifier in which the circuit is grounded onone side, to an amplifier stage which is balanced or symmetrical withrespect to ground.

The connection of such amplifier circuits may be readily accomplished bymeans of transformers but in cases where transformers are objectionableon the ground of frequency limitations, as where very low or very highfrequencies are to be amplified, the problem is more difiicult.

The object of this invention is to provide an arrangement of amplifiercircuits for accomplishing the above mentioned purpose, wherein the twoelectron tubes of a balanced or push-pull stage are excited by voltagesof exact equality but of opposite phase.

In the accompanying drawing I have illustrated in Figs. 1 to 4 inclusivedifferent modifications embodying this invention.

Referring to Figure 1, tubes C and D comprise the balanced output stageof the amplifier, which require the grids to be supplied with excitingvioltages equal in magnitude but opposite in phase. The grid of the tubeC is coupled to the output of amplifier tube A of the unbalanced orgrounded circuit and is directly excited by signal voltage in the normalmanner. The exciting voltage for the grid of the other tube D, of thepush-pull stage, is derived through the agencyof an auxiliary phaseinverter tube B. The grid of tube B, however, receives its excitationfrom the resistance R1, which reflects the potential occurring acrossthe self-bias resistance R2 included in series with the cathode circuitof the output stage C, D. A condenser C1, acts as a blocking condenserto isolate the grid of tube B from the cathode potential of the twooutput tubes, With this arrangement no degenerative action can takeplace in the operation of the inverter tube B.

When the two tubes of a push pull amplifier are properly matched, nosignal component appears in the common portion of the circuit whichincludes the resistance R2. The potential across this resistance shouldbe constant and is due to the steady component of the plate current ofthe tubes C and D. Now if an A. C. potential should appear across thisresistance R2, due to any unbalance between the tubes C and D, thispotential.

will also exist across resistance R1 and be impressed upon the grid oftube B, which will then impress via its plate circuit, a compensatingpotential upon the grid of tube D. Thus the tube D tends to continuouslyneutralize any tendency of tube C to produce an alternating componentacross the resistance R2 and hence maintains a zero A. C. potentialacross the said resistance. The phase inverter is thus selfbalancing inits action and renders unnecessary a high degree of symmetry in thetubes and the couplin elements.

The magnitude of the voltage applied to the grid of tube D can be madeto match that applied to the opposing tube through regulation of thegain of tube B. Ordinarily this tube should be of a high gain type. Infact, a multi-stage amplifier could be used in this position, so long asthe phase of the potential applied to tube D is maintained 180 from thatapplied to tube C. That the voltage applied to the grid of tube B is 180degrees out of phase withthat applied to tube A is evident as it is wellknown that a complete phase reversal is present between successivestages in resistance coupled amplifiers.

The other features of this amplifier are conventional and are familiarto engineers and those skilled in this art. Transformer coupling to bothinput and output of the amplifier have been shown, but direct couplingmay also be employed and furthermore, transformer coupling may be usedfor any intermediate stages. This amplifier circuit possesses aparticular advantage in that the signal or A. C. currents in the platecircuit of the two output tubes C and D are maintained continuouslyequal. This is the condition for neutralization of the A. C. componentin the resistance R2, and is accomplished through the functioning of thephase inverter tube B. In this manner the distortion which so frequentlyoccurs when push pull tubes are imperfectly matched is avoided.

The element R2 has been indicated as a resistance but it may toadvantage be an inductance. An inductance would serve for example toprovide an extraordinarily high degree of balance at high frequencieswhile leaving the D. C. relations undisturbed.

Figure 2 differs from Figure 1 only in that the tube A has been omitted.The gain due to this tube is therefore sacrificed but in all otherrespects the functioning of the amplifier is identical with that ofFigure l. The amplifier coupling condenser and resistance associatedwith tube A in Fig. l have been retained in Fig. .2. In

a general, however, these elements may be dis pensed with in the circuitof Fig. 2.

Figure 3 illustrates another modification of the invention showing astage of an amplifier provided with tubes in push-pull arrangement,having the advantage of being conductively coupled throughout and isadapted to function with the positive and negative grounded batteriesnormally available in telegraph ofiices. The tubes C and D comprise abalanced direct coupled amplifier stage excited from a single groundedsource, viz., condenser C2.

Assume that an impulse of positive polarity is received over the line L,thereby impressing a positive control potential upon condenser C2 andthe grid of tube C. The plate current of the tube C will be increasedand since this current must flow through resistance R2, the cathode ofthis tube will be made more positive with respect to ground. The platecurrent of tube C will re main large under the influence of the positivesignal, but the plate current of tube D will diminish since the grid ismore negative than the cathode. Hence the grid of tube C will bepositive while the grid of tube D will be negative with respect to theircathodes and the respective plate potentials of the two tubes will havethe opposing positive and negative values corresponding to normal pushpull operation. The gain of tube C will undergo some reduction, but thisreduction will not exceed that which normally occurs in push pull tubeswhere the input signal is divided between two tubes. Generally theresistance R2 will be rather high in value to produce proper balancedoperation. The battery on is therefore added for the purpose ofrestoring the grid potentials of the two tubes to the proper position onthe tube operating characteristic.

The arrangement illustrated in Fig. 4 shows an amplifier which combinesfeatures of Figs. 1 and 3, together with certain alterations to providea conductively coupled amplifier possessing the advantages of thearrangement shown in Fig. 1. Batteries b1 and be having the mid-pointgrounded, provide positive plate potential for the tube B, and alsoprovide an appropriate negative potential for the grid of tube D. Sincea condenser is not permissible in the grid circuit of the tube B, abiasing battery b3, is included in this circuit to provide anappropriate bias for tube B. The output circuit corresponds to thatshown in Fig. 3 but it may be altered in various ways familiar toengineers and persons versed in amplifier principles.

In the conductively coupled amplifier of Fig. 4, equality of the platecurrents of the tubes C and D is obtained by adjustment of thepotentiometer P. Exact equality of normal or D. 0. plate currents is notimportant in the amplifiers of Figs. 1, 2, and 4, since the sum of thesetwo currents serves only to provide the continuous selfbias in theresistance R2, while the signal or A. C. components vary upward ordownward from this steady bias as a mean value.

While I have shown several embodiments of my invention, it will beunderstood that I do not wish to be limited thereto, since otherarrangements and modifications of amplifiers with phaseinverter circuitsembodying the principles disclosed herein may be employed withoutdeparting from the scope of my invention as defined in the appendedclaims,

I claim:

1. In a signal wave transmitting apparatus, a push-pull amplifiercomprising two electron tubes having an input circuit including portionsindividual to said tubes and a grounded circuit common to the cathodesof said tubes including a self-bias resistance, an unsymmetricallygrounded line, a non-inductive coupling means for connecting the line tothe input of one tube, without substantial phase change and means forconnecting the line to the input of the other tube with interposed phasereversal, said means including an auxiliary electron tube having itsinput connected so as to be actuated by any signal voltage componentsoccurring between said cathodes and ground and having its outputconnected to the input of the other of said tubes, to thereby counteractany tendency to unbalance between said tubes.

2. In a signal wave transmitting apparatus, a push-pull amplifiercomprising two electron tubes having an input circuit including portionsindividual to said tubes and a grounded circuit common to the cathodesof said tubes including a self-bias resistance, an unsymmetricallygrounded line, a non-inductive coupling means for connecting the line tothe input of one tube without substantial phase change and an auxiliaryphase reversing electron tube having its cathode grounded, its outputconnected to the other of said tubes and its input connected to receiveany signal voltage components occurring between the oathodes of saidtubes and ground but free from any continuous potentials in said commoncathode circuit and thereby operating to nullify any unbalance in theoutput of said tubes.

,3. In a signal transmitting system, a push-pull amplifier stage forconnecting a non-symmetrical circuit to a symmetrical circuit,comprising two electron tubes in'push-pull relation, having individualinput circuits and a common grounded cathode circuit, a signaling linehaving one side grounded and its other side connected to the input ofone of said tubes, an auxiliary phase-inverting electron tube connectedto said line and having its input connected to receive any potentialvariations occurring between ground and the cathodes of said push-pulltubes and its output connected to the input of said other electron tube,said phase inverting tube thereby operating to maintain said push-pulltubes in balanced relation.

4. In a signal repeating system, a push-pull stage comprising twoelectron tubes having input circuits and having a common groundedcathode circuit containing an impedance, a signaling line having oneside grounded and having its other side connected to the input circuitof one of said tubes, an auxiliary phase reversing electron tube havingits cathode grounded, its output connected to the input of the other ofsaid tubes, and having its input operatively connected to receivepotential variations caused by signal components occurring in saidcommon cathode circuit, to thereby counteract any unbalance between thetubes of said push-pull stage.

WILLIAM D. CANNON.

